NAND flash memory has been developed as a large-capacity nonvolatile memory device. In NAND flash memory, data is stored by storing charge in floating gate electrodes. However, interference between the floating gate electrodes becomes problematic when the cell structure of NAND flash memory is shrunk to increase the bit density. Although a method has been proposed in which a portion of the control gate electrode is extended into the space between the floating gate electrodes to suppress the interference between the floating gate electrodes, in such a case, the distance between the silicon substrate and the control gate electrode becomes short; and the breakdown voltage between the silicon substrate and the control gate electrode undesirably decreases.